The last time I used my wart chiller I made the mistake of leaving it in a weak bleach cleaning solution overnight. When I found it the next day the bleach had corroded a few small spots just through the outer skin of the copper. Can I still use it or is it time to buy a new one?

I am not sure I have a clear answer for you because without seeing the chiller it is hard to say whether or not I would use it. I have read however that lemon juice can be used in cleaning corroded copper (I have never done this and am uncertain of the results). There are also other chemical products that can be used for removing copper corrosion (I don't know their names). If you were able to clean off the corrosion and it wasn't much to begin with I would not worry so much about using it. Although if the corrosion remains even after cleaning I would replace it. Here is a link to an article that addresses the use of copper in brewing as well as cleaning copper and may help you to make your decision: